Electric heating methods for heat treatment include induction heating and direct electric heating. In hardening treatment in particular which is one kind of heat treatment using induction heating, a proper frequency is selected according to a heat treatment depth in a workpiece.
According to a related art power conversion apparatus for heat treatment, DC power is converted into high-frequency power by performing switching using a power semiconductor device (see, e.g., pamphlet of MK16A Transistor Inverter, Neturen Co., Ltd., www.k-neturen.co.jp/eng/business/induction/pdf/MK16Aenglish.pdf). Examples of the power semiconductor switching device includes a thyristor for lower than 10 kHz, an IGBT for 10 kHz to 100 kHz, and a MOSFET for higher than 100 kHz.
Comparing the oscillation frequency of 10 kHz and the oscillation frequency of 100 kHz, the power semiconductor device undergoes much different temperature increase because of the 10 times difference in the switching frequency. That is, if a capacity (maximum rated value) of an inverter of the power conversion apparatus is set based on the maximum frequency of an operable range of the apparatus, the temperature increase is small when the output frequency is low and hence the operation is not economical.
In a power conversion apparatus having a power semiconductor device as described above, the temperature is controlled such that the junction temperature of the power semiconductor device does not exceed a given temperature. More specifically, a thermostat is attached to the periphery of the power semiconductor device or a thermistor is incorporated in the power semiconductor device. Output of power is controlled or suspended only after the actual temperature has reached the given temperature.
Another related art power conversion apparatus includes a semiconductor device for performing a power conversion, heat radiation fins for radiating heat generated by the semiconductor device, a cooling fan for cooling the radiation fins, a detection unit for detecting a parameter relating to the cooling performance of the cooling fan, and a control unit (see, e.g., JP2012-39745A). In this power conversion apparatus, a junction temperature of the semiconductor device is estimated based on a detection result of the detection unit, a loss of the semiconductor device, and an ambient temperature of the semiconductor device, and the semiconductor device is controlled such that the estimated junction temperature does not exceed a given temperature.
However, according to the temperature controlling method described above, because the output of power is suspended only after the actual temperature has reached the given temperature, a rapid temperature increase cannot be addressed due to a response delay of a temperature sensor, and may lead to a damage of the semiconductor device. According to the related art power conversion apparatus described above, the semiconductor device is air-cooled, and the junction temperature of the semiconductor device is estimated taking into account the parameter relating to the air cooling performance. Further, the temperature sensor is attached to the periphery of the semiconductor device, and the junction temperature of the semiconductor device is estimated based on the temperature detected by the temperature sensor. However, the measured temperature value largely varies depending on the position at which the temperature sensor is attached. Accordingly, this related art cannot control the semiconductor device with sufficient accuracy.